I have a different perspective because I was an overweight child and while I won't say that I completely understand your stepson's situation, I do know that "forcing" him to diet is only going to cause him to rebel. He'll start to sneak food, if he's dead set against the "diet".
Does he have a role model, or a hero, or an athlete that he looks up to that plays sports? Maybe focusing on that person and how they eat would be helpful to get him to a new way of thinking about food.
I understand it's a very difficult situation for you because his mom is in denial about this. So let me ask you: can you guys make your time with him more active? Take him for walks, hikes, bike rides, play basketball - do something to get him moving - try to teach him that being active is fun.
If I could have had someone that cared about overweight me when I was his age, I would have been so grateful because it would have saved me years of struggle and an eating disorder. Your wanting to help him is a beautiful thing.
Does your husband have health benefits for the child that make allowances for family therapy? Could it be that if you all got counseling together with his mom, that maybe an impartial 3rd party could help her to see/get involved with her son's health?
Mc'D's does have some "healthier" choices, such as salads (just gotta watch the dressings they give you for those!) and yogurt and fruit parfaits - maybe getting him active and showing him that those choices give him more energy than the "junk" can help him?
I don't know if I'm expressing it well, but the whole "diet" mentality isn't going to work - maybe focusing on just healthy eating for the family as a whole with no emphasis can help him see a difference when he's with your family. He will feel a difference physically when he eats well vs. tired and cruddy when he eats McD's fried stuff.
Hopefully this will help at least a little bit. Keep us posted.